Question: Divergencia y rotacional
Encuentra la divergencia y el rotacional de los siguientes campos vectoriales: $F(x, y, z)=\cos xz \mathbf{j} - \sin xy \mathbf{k}$
1. **Planteamiento del problema:**
Queremos encontrar la divergencia y el rotacional del campo vectorial $$F(x,y,z) = 0\mathbf{i} + \cos(xz)\mathbf{j} - \sin(xy)\mathbf{k}$$.
2. **Fórmulas importantes:**
- La divergencia de un campo vectorial $\mathbf{F} = P\mathbf{i} + Q\mathbf{j} + R\mathbf{k}$ es $$\nabla \cdot \mathbf{F} = \frac{\partial P}{\partial x} + \frac{\partial Q}{\partial y} + \frac{\partial R}{\partial z}$$
- El rotacional de $\mathbf{F}$ es $$\nabla \times \mathbf{F} = \left( \frac{\partial R}{\partial y} - \frac{\partial Q}{\partial z} \right) \mathbf{i} - \left( \frac{\partial R}{\partial x} - \frac{\partial P}{\partial z} \right) \mathbf{j} + \left( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right) \mathbf{k}$$
3. **Identificamos componentes:**
$$P = 0, \quad Q = \cos(xz), \quad R = -\sin(xy)$$
4. **Calculamos la divergencia:**
$$\frac{\partial P}{\partial x} = \frac{\partial 0}{\partial x} = 0$$
$$\frac{\partial Q}{\partial y} = \frac{\partial}{\partial y} \cos(xz) = 0 \quad \text{(porque no depende de } y)$$
$$\frac{\partial R}{\partial z} = \frac{\partial}{\partial z} (-\sin(xy)) = 0 \quad \text{(porque no depende de } z)$$
Por lo tanto,
$$\nabla \cdot \mathbf{F} = 0 + 0 + 0 = 0$$
5. **Calculamos el rotacional:**
- Componente $\mathbf{i}$:
$$\frac{\partial R}{\partial y} = \frac{\partial}{\partial y} (-\sin(xy)) = -\cos(xy) \cdot x$$
$$\frac{\partial Q}{\partial z} = \frac{\partial}{\partial z} \cos(xz) = -\sin(xz) \cdot x$$
Entonces,
$$\left( \frac{\partial R}{\partial y} - \frac{\partial Q}{\partial z} \right) = -x \cos(xy) - (-x \sin(xz)) = -x \cos(xy) + x \sin(xz)$$
- Componente $\mathbf{j}$:
$$\frac{\partial R}{\partial x} = \frac{\partial}{\partial x} (-\sin(xy)) = -\cos(xy) \cdot y$$
$$\frac{\partial P}{\partial z} = \frac{\partial 0}{\partial z} = 0$$
Entonces,
$$- \left( \frac{\partial R}{\partial x} - \frac{\partial P}{\partial z} \right) = -(-y \cos(xy) - 0) = y \cos(xy)$$
- Componente $\mathbf{k}$:
$$\frac{\partial Q}{\partial x} = \frac{\partial}{\partial x} \cos(xz) = -\sin(xz) \cdot z$$
$$\frac{\partial P}{\partial y} = \frac{\partial 0}{\partial y} = 0$$
Entonces,
$$\left( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right) = -z \sin(xz) - 0 = -z \sin(xz)$$
6. **Resultado final:**
$$\nabla \cdot \mathbf{F} = 0$$
$$\nabla \times \mathbf{F} = \left(-x \cos(xy) + x \sin(xz)\right) \mathbf{i} + y \cos(xy) \mathbf{j} - z \sin(xz) \mathbf{k}$$